1. Field of the Invention
The invention pertains to the field of telecommunications, in particular to broadband integrated services digital networks or "broadband ISDN", namely multiple service networks with a wide range of throughput rates, which may extend from the transmission of high-definition digital television at 150 megabits per second to the transmission of short spoken messages, for example for civilian or military applications, where the speech is encoded by linear predictive coding, for example LPC10, using 2400 bits/s vocoders.
2. Description of the Prior Art
For broadband integrated services digital networks, an international conference has chosen the asynchronous mode for the transfer of data, namely a transfer in cells of fixed lengths, comprising a header and a useful data field. The header has a length of 3 to 5 eight-bit bytes while the useful data field will have a length ranging from 32 to 120 eight-bit bytes. Once the characteristics of the network have been defined, these lengths will be fixed. The final decision on the respective lengths of the header and the useful data field should give the most advantageous compromise for the transmission quality to be acceptable, with delays and losses of information being reduced to a minimum. To optimize the transmission efficiency, the ratio between the size of the header and the size of the useful data field should enable the proper transmission of all the services envisaged. But irrespectively of the final values chosen, it is clear that the transmission efficiency will not be at a maximum for links with low throughput rates, given the fact that the cells will not be completely filled for this type of transmission. If l.sub.e is the length of the header, and if l.sub.c is the length of the data field, u being the length of the useful information transmitted in a cell, the efficiency of the transmission function is measured by e=u/(l.sub.c +l.sub.e). For low throughput rate data such as speech encoded in 2400 bits/s, u=4 eight-bit bytes, i.e. the packaging will correspond to a duration of speech of 14 milliseconds. If we use a standard cell such as the one shown in FIG. 1, wherein the length of the header l.sub.e is equal to 3 eight-bit bytes, and wherein l.sub.c is equal to 32 eight-bit bytes (the minimum length stipulated in the international recommendation), the transmission efficiency for this type of data is 12%.
Consequently, a network in which the transfer of data is done in an asynchronous mode actually enables these different types of information to be transmitted in cells of fixed length, the information being distributed in a sequence of cells, when necessary at high throughput rates, the cells being filled only partially, when necessary at low throughput rates. However, for low throughput rate links, a cell is transmitted and then routed through the different junctions of the network only when it is entirely received at the network. This leads to a transmission efficiency which is lower than that obtained when the cell is completely filled, owing to the "packaging" time. In other words, the transmission of data at a low throughput rate is done with a certain delay.
For certain types of services, this kind of transfer entails no drawbacks. On the other hand, this transfer in an asynchronous mode may be troublesome for low throughput rate links wherein transmission in real time is desired, for example for tactical links or for links useful in aeronautics.